The instant invention lies generally in the field of spectroscopy, and specifically in the field of Raman scattering spectroscopy.
Molecular spectroscopy is a family of analytical techniques that provide information about molecular structure by studying the interaction of electromagnetic radiation with the materials of interest. In most of these techniques, the information is generally obtained by studying the absorption of radiation as a function of optical frequency.
Raman spectroscopy is unique in that it analyzes the radiation that is emitted (or scattered) when the sample is irradiated by an intense optical signal consisting of a single frequency, or a narrow range of frequencies. FIG. 1 is a simplified view of a typical application of Raman spectroscopy. As shown, a laser source 10 outputs an excitation beam 11 that intensely irradiates a sample that is contained within a sample cell or a flow cell 12. Note that the laser""s excitation radiation 11 is shown as a solid line. As further shown by dashed lines 13, some Raman scattering of variously shifted wavelengths occurs due to the laser light""s Raman interaction with the sample""s molecular bonds. As shown by more solid lines 14, however, a great deal more Rayleigh scattering occurs at the original laser frequency due to the laser""s interaction with the atoms as opposed to with the bonds. In the typical system, a long pass laser rejection filter, or simply blocking filter 20, blocks the Rayleigh scattering (solid lines 14) while passing the Raman scattering (dashed lines 13, 15) on to a spectrometer 31 for detection and analysis (typically with the assistance of a separate general purpose computer 32 as shown).
As described above, the xe2x80x9cRaman scatteringxe2x80x9d signal is essentially an emission spectrum with frequency dependent intensities. The individual bands in this spectrum are shifted from the frequency of the excitation signal by amounts that are related to the structure of the molecules present in the sample.
Modern Raman spectrometers typically use powerful single wavelength lasers as the source of excitation radiation. Nonetheless, Raman scattering is an extremely rare event, so very little of the laser radiation is actually converted to Raman shifted energy. Most of it simply travels on through the sample without interaction, or is Rayleigh scattered by the sample without having its frequency altered. The weakness of the Raman signal is in part offset by the very high sensitivity of the visible and infrared detectors used. However, it is important to minimize the amount of reflected or Rayleigh scattered laser radiation that gets into the receiving optics since this can swamp the detector either on its own or by generating fluorescence and/or Raman radiation in the receiving optics.
Most sample interfacing systems currently being used in Raman spectroscopy employ optical filters to separate the excitation signal from the Raman shifted signal being studied. Some examples are given in I. R. Lewis and P. R. Griffiths, xe2x80x9cRaman Spectroscopy with Fiber-Optic Samplingxe2x80x9d, Applied Spectroscopy. Vol. 50, pg. 12A, 1996, FIGS. 5 through 11. (Also see U.S. Pat. Nos. 5,112,127 and 5,377,004.) In most of these designs, a dichroic filter inclined to the axis of the optical path combines the transmitted and received paths. In at least one case (FIG. 5,b of Ref. 1), the transmitted and received paths are inclined at an angle to each other.
Mirrors have been proposed for enhancing Raman radiation signal levels in Raman Spectroscopy, but the enhancement provided by the embodiments known to the inventor do not provide as much gain as is possible with the present invention. In xe2x80x9cRaman Spectroscopy for Chemical Analysisxe2x80x9d, Wiley Interscience, New York, N.Y. 2000, pg. 121, for example, Richard L. McCreery reported an enhancement in the collected signal by providing two passes of excitation radiation by reflecting the excitation radiation back through the sample. The enhancement of signal levels is due to two factors: (1) the increased laser radiation intensity caused by folding the beam back through the sample; and (2) the fact that the mirror facilitates the collection of forward scattered Raman radiation as well as back scattered radiation. While signal levels have been improved by this method, the inventor has found that it is less than double. The enhancement is reduced by the fact that only half of the focal region is available for interaction of the beam with the sample.
There remains a need, therefore, for a Raman spectroscopy system that provides significantly increased Raman radiation signal levels beyond those currently available.
FIGS. 2-4 depict three different versions 50, 60, 70 of the RFP-400 Series Raman probes manufactured by the inventor and disclosed in the provisional application referred to above. As shown, all three probes 50, 60, 70 are based on a unique design in which the transmitted excitation beam 11 is injected along a path which is parallel to the received collection beam 15 by means of a small reflecting optical element such as a mirror or rhomboid 54, 64, 74. As a result, the long pass filter 57, 67, 77 used to eliminate the laser signal from the receiving optics can be perpendicular to the path 15. The illustrated probes 50, 60, 70 are not multi-pass probes. The characteristics of the RFP-400 design shown in FIGS. 2-4, however, pave the way for the unique family of enhancements that are the subject of this application.
The preferred embodiments of the present invention take advantage of the facts that:
1. The difference between the laser excitation frequency and the frequencies of the Raman shifted radiation;
2. The fact that Raman scattering events are rare whereby very little excitation radiation is lost on each pass through the sample;
3. The fact that most interference filters are highly reflecting at frequencies there they block the laser signal from being transmitted; and
4. The fact that many samples are highly transparent in the frequency range characteristic of the Raman shifted radiation.
Each of the embodiments includes the following elements:
1. The use of a laser frequency blocking filter approximately perpendicular to the axis of the optical system;
2. A lens or other focusing device positioned in the common transmitted and received optical path so as to focus the laser radiation into a small region within the sample and, at the same time, collect Raman scattered radiation from this region;
3. A mirror within or behind the sample to be analyzed positioned so as to reflect the transmitted laser signal back through the sample so that at least a portion of it strikes the blocking filter at approximately normal incidence;
4. A means for injecting the laser signal into the optical path between the blocking filter and the lens so as to obscure only a minor portion of the receiving optical path.
The invention may be regarded as a multi-pass Raman sampling system that illuminates a sample with laser excitation radiation to produce Raman shifted radiation, the system comprising: an injection element located in an optical path, the injection element obscuring only a portion of the optical path while injecting a substantially collimated beam of laser excitation radiation toward the sample in a first direction; an objective lens positioned in the optical path and defining an optical axis and a focal point, the objective lens focusing the collimated beam of laser excitation radiation traveling in the first direction into the sample to produce Raman shifted radiation, the objective lens collecting radiation that is emanating from at or near the focal point and then transmitting the radiation into the optical path in a second direction that is substantially opposite to the first direction; a mirror located within or behind the sample at or near the focal point, the mirror reflecting both laser excitation radiation and Raman shifted radiation back through the sample toward the objective lens and the injection element in the second opposite direction; a blocking filter located optically beyond the injection element such that the injection element is in the optical path between the objective lens and the blocking filter, the blocking filter being substantially reflective to the laser excitation radiation and substantially transparent to the Raman shifted radiation, the blocking filter passing the Raman shifted radiation out of the system for analysis and reflecting the laser excitation radiation back in the first direction toward the objective lens, the sample, and the mirror; and signal enhancement means for causing at least a portion of the laser excitation radiation that is reflected by the mirror to miss the injection element and strike the blocking filter where it is reflected back to the sample through the objective lens to produce more Raman shifted radiation. The just summarized invention is best understood with reference to the following drawings taken together with the accompanying description.